Force microscopy experiments with ultrasensitive cantilevers

Abstract

Force microscopy experiments with the pendulum geometry are performed withattonewton sensitivity (Rugar et al 2004 Nature 43 329). Single-crystallinecantilevers with sub-millinewton spring constants were annealed underultrahigh-vacuum conditions. It is found that annealing with temperatures below500 °C can improve the quality factor by an order of magnitude. The high force sensitivity of theseultrasoft cantilevers is used to characterize small magnetic and superconductive particles,which are mounted on the end of the cantilever. Their magnetic properties areanalysed in magnetic fields as a function of temperature. The transition of asuperconducting sample mounted on a cantilever is measured by the detection of frequencyshifts. An increase of dissipation is observed below the critical temperature. Themagnetic moment of ferromagnetic particles is determined by real time frequencydetection with a phase-locked loop (PLL) as a function of the magnetic field.The dissipation between the probing tip and the sample is another important ingredient forultrasensitive force measurements. It is found that dissipation increases at separations of30 nm. The origins of this type of dissipation are poorly understood. However, it ispredicted theoretically that adsorbates can increase this dissipation channel (Volokitin andPersson 2005 Phys. Rev. Lett. 94 086104). First experiments are performed under ultrahighvacuum to investigate this type of dissipation. Long-range dissipation is closely related tolong-range forces. The distance dependence of the contact potential is found to be animportant aspect.